Ive played perhaps 5 games of dominion out of my 2000+games played, and personally i think its good you are not rewarded EXP for dominion for the sole fact that you dont become 30 off of dominion and then go play Ranked 5v5 summoner rift solo matches and bring your team down because of not understanding the meta or map itself.

That being said XD If your going to buy your XP boost, you might as well buy a IP boost with it. (No im not a promoter for the LoL store) but the sole reason i say to do this is because with a XP boost you are leveling twice as fast as other people, but those players you are now playing against at lvl 20-25 will have more champions/ better runes because they will have played more games and earned more IP than you so in the end it wont be much fun for you when you are playing a caitlyn or vayne and are going up against a graves/ezreal that is already starting out with +20 AD/AP over you.. A stat like that is Huge at lvl 1-6.

However, If your sole purpose is to get to lvl 30 without really caring about your accounts respectability itself then i would XP Boost the 3 day as you said if you are going to be playing 8+hours a day and then play only 5v5 summoner rifts and a occasional 3v3 Tree Line when you are bored or aggravated with the lower level players.. The thing about treeline and summoners is that the treeline can go just as long as a summoners rift which could easily go to 40mins. Now if you are a smurf then it would be different and i would suggest getting 2 buddys and playing treelines up to lvl 20 and win 15min games... If your solo queue then play summoners rifts, you will usually have teams FF(Surrender) at 20mins if they are losing 15-3 and you will get more EXP than you wouldve gotten from a 40min treeline game.